This invention relates to a process for the production of sugar thick juice from lime-containing raw sugar juice, the method comprising the steps of subjecting the lime-containing raw sugar juice to at least one treatment with carbon dioxide and soda to precipitate calcium salts, separating the precipitated calcium salts from the juice to form a thin juice, preheating the thin juice thus obtained and evaporating the thin juice to form a thick juice, the gaseous effluent obtained during the evaporation of the preheated thin juice being used as an indirect heating medium for the preheating of the thin juice.
In the conventional manufacture of crystalline sugar from sugar juice obtained by extraction of sugar beet slices, the extraction juice is purified in a liming step followed by at least one and ordinarily two carbonation steps. The liming, which comprises the addition of lime to the extraction juice has a dual purpose, namely of forming precipitates comprising non-sugars and calcium oxide, which precipitates subsequently are separated by filtration, and of decomposing such compounds, such as amino acids, glucose and fructose, which do not form precipitates when reacted with calcium oxide and which adversely affect the further sugar production steps, unless they are decomposed.
The carbonation, which ordinarily includes two steps, comprises the addition of carbon dioxide to the limed sugar juice in order to precipitate excessive amounts of lime in the form of calcium carbonate which subsequently is removed by filtration. Since it has been found that an insufficient alkalinity of the juice or an inefficient extraction which may occur at the end of the sugar beet campaign makes it difficult to remove excessive amounts of lime during the second carbonation, it is normal practice to supplement the addition of carbon dioxide during the carbonation, and particularly during the second carbonation, with an addition of soda. The soda consumption constitutes a significant expenditure in the purification of sugar juice. Furthermore, the sodium ions thus introduced have an adverse effect on the sugar crystallisation step and cause sugar to be transferred to the molasses, thus reducing the sugar yield.
The sugar juice obtained when the calcium carbonate formed during the second carbonation has been removed by filtration may optionally be subjected to a further treatment with sulphurous acid. The treatment with sulphurous acid partly causes the pH value of the juice to be adjusted and partly to reduce the discolouration in the evaporator. Due to the adjustment of the pH value, the decomposition of saccharose in the evaporator is minimized.
The purified thin juice is then preheated before it is evaporated to form thick juice. The thick juice is introduced into the boilers in which it is further evaporated to effect a crystallization of sugar crystals which are separated and worked up.
The preheating is ordinarily effected in a row of preheaters connected in series, and a similar number of evaporators also connected in series is used for the evaporation of the thin juice. The evaporation of the thin juice is effected, e.g., by using steam formed in a preceding evaporator in the row of evaporators as heating medium in the following evaporator and by indirectly using spent heating medium from one evaporator as heating medium in a heat exchanger in the corresponding preheater in the row of preheaters. Up to now the gaseous heating medium from the heat exchangers in the preheaters has been discharged to the atmosphere, optionally through the vacuum system of the sugar factory.